This invention relates to a high-voltage semiconductor device, and more particularly to a lateral high-voltage semiconductor device with high breakdown strength at the corners of the field plates.
In conventional high-voltage semiconductor elements used for power ICs, electrodes are provided in the same plane. To form an IC by connecting each element, the electrodes have to be connected to one another.
In general, a field plate is provided in an area between the high-voltage section and the low-voltage section via a field oxide film to alleviate the electric field at the surface of the semiconductor device.
A thick field oxide film is required to achieve high breakdown strength. The thick field oxide film makes the processing very difficult and the breakdown strength may be deteriorated due to a reliability test.
To overcome the problems, a method has been tried. In the method, a high-resistance film, such as an SIPOS film, is used to produce a uniform electric field in the lateral direction of the field oxide film. Leakage current is allowed to flow over the film. Use of such a high-resistance film, however, causes a rise in the cost and a reliability problem.
To avoid the problems, another method has been considered. In the method, a high-resistance silicon film is used as a field plate and made as narrow as possible. The narrow film connects the anode and cathode of each element. FIG. 6 is a plan view of a conventional high-voltage semiconductor device using this type of field plate. FIG. 7 is a sectional view taken along line VII--VII of FIG. 6. FIGS. 6 and 7 show a lateral diode as an example of high-voltage semiconductor device.
In the figures, numeral "1" indicates a substrate, "2" indicates an SiO.sub.2 film, "3" indicates a high-resistance n-type cathode layer formed in a silicon substrate (SOI). At the surface of the n-type cathode layer 3, a p-type anode layer 4 and an n-type cathode layer 5 are selectively formed. At the surfaces of the p-type anode layer 4 and n-type cathode layer 5, a high-impurity-concentration p-type contact layer 6 and a high-impurity-concentration n-type contact layer 7 are selectively formed, respectively.
On the surface of the n-type cathode layer 3 in the area between the p-type anode layer 4 and the n-type cathode layer 5, a LOCOS oxide film 8 is formed. On the LOCOS oxide film 8, a spiral field plate 9 composed of a polysilicon film is formed.
The field plate 9 is composed of a track-like first field plate 9.sub.1 connected to an anode electrode 10, a track-like second field plate 9.sub.2 which is surrounded by the first field plate 9.sub.1 and is connected to a cathode electrode 11, and a spiral field plate 9a one end of which is connected to the first field plate 9.sub.1 and the other end of which is connected to the second field plate 9.sub.2. Numeral "12" indicates an interlayer insulating film.
To produce a uniform electric field needed to alleviate the electric field, the spiral pitch of the field plate 9a is set narrow. When the element width W becomes greater and the total length of the field plate 9 becomes larger, leakage current decreases in the field plate 9. If spiral field plate 9a is particularly narrow, almost no leakage current flows. As a result, it is difficult to maintain the uniform electric field, degrading the breakdown strength.
In a lateral semiconductor device having the above-mentioned structure, the electrodes connected to the semiconductor region in the central portion of the device are provided in such a manner that they cross the other regions. Because of the effect of an electric field caused by the currents flowing through the electrodes, a breakdown is liable to take place at and near the electrodes, which is a problem peculiar to the lateral semiconductor element.